1. Field of the Invention
The present invention relates to a Complementary Metal Oxide Semiconductor (CMOS) image sensor and a method of operating the same. More particularly, the present invention relates to a CMOS image sensor with improved dark current characteristics and a method of operating the same.
2. Description of the Related Art
Image sensors have been applied in various fields such as machine vision, robotics, satellite-based instrumentation, transportation, navigation, and guidance. Typically, image sensors have a two-dimensional array of pixels constituting an image frame.
Pixels include photoelectric conversion elements capable of accumulating charge corresponding to the amount of light absorbed. That is, when photons collide with surfaces of photoelectric conversion elements formed on a semiconductor substrate, free charge carriers are generated and collected in the photoelectric conversion elements. The collected carriers are read out and then transferred to an output circuit through various procedures, thereby reproducing images.
Representative image sensors are Charge Coupled Devices (CCDs) and Complementary Metal Oxide Semiconductor (CMOS) image sensors. The CCDs have a lower noise level and produce better image quality than the CMOS image sensors. However, the CMOS image sensors can be easily operated and adopt various scanning techniques. Furthermore, signal processing circuits can be integrated on a single chip, thus enabling miniaturization of products, and the compatibility with CMOS processing can reduce manufacturing costs. In addition, CMOS image sensors are very low in power consumption, and thus, can be easily adopted in products with limited battery capacity. Thus, CMOS image sensors are in widespread use.
The CMOS image sensors can be fabricated in various structures, but generally have a structure including four transistors and a photodiode, which is called a “4Tr structure” hereinafter. The 4Tr structure is manufactured using a common CMOS manufacturing process. However, the 4Tr structure generates a relatively large dark current.
A dark current is an output signal generated even when an object is not exposed to a pixel array unit. That is, the dark current is noise. It is generated regardless of whether there is an image to be recorded.
Excessively large dark current renders discrimination of brightness and darkness difficult, thereby decreasing a dynamic range. Further, a non-uniform and excessive dark current causes a “white defect”, which generates a larger signal than normal. Therefore, the development of techniques to reduce the dark current is desirable.